Distillation column design & optimization

Distillation is a process that separates two or more components into an overhead distillate and bottoms. The bottom product is almost exclusively liquid, while the distillate may be liquid or a vapor or both.

The separation process requires three things.

  • First, a second phase must be formed so that both liquid and vapor phases are present and can contact each other on each stage within a separation column.
  • Secondly, the components have different volatilities so that they will partition between the two phases to different extent.
  • Lastly, the two phases can be separated by gravity or other mechanical means.

Distillation differs from absorption and stripping in that the second phase is created by thermal means

Distillation Flow:

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The distillation column consists of:

  • 1 feed stream – which consistsof a mole percent of the light component, Zf.
  • 2 product streams – exiting the topwhich has a composition of Xd of the light component. The product stream leaving the bottomcontains a composition of Xb of the light component.

The column is broken in two sections:

  • The top section referred to as the rectifying section – which passes through a total condenser to condense all of the vapor distillate to liquid.
  • The bottom section isknown as the stripping section – which uses a partial reboiler to allow for the input of energy into our column.

Applications of Distillation:

  • Distillation is the least expensive means of separating mixtures of liquids.
  • If relative volatilities of components is less than 1.1, distillation becomes very expensive & extraction or reactive distillation should be considered.

What do successful distillation scale-up projects have in common? Project managers

Distillation column designmust understand and determine five key design elements for project success. Cost, chemical interactions and equipment needs change in a non-linear fashion as increased output is required.

STEPS INVOLVED IN COLUMN DESIGN:

  • SECTION 1:Graphical Determination of a Distillation Column Design

    Step 1. Determine Process Operation Variables
    Step 2. Determine the Minimum Reflux Ratio
    Step 3. Choose Actual Reflux Ratio
    Step 4. Determine the Minimum Number of Trays
    Step 5. Determine Actual Number of Trays
    Step 6. Principal Dimensions of the Column (Diameter/Height)

  • SECTION 2:Analytically determining the specifications for a distillation column

    Step 1. Determine Process Operation Variables
    Step 2. Determine the Minimum Reflux Ratio
    Step 3. Choose Theoretical Reflux Ratio
    Step 4. Determine Theoretical Number of Trays
    Step 5. Determine Actual Number of Trays
    Step 6. Principal Dimensions of the Column (Diameter/Height)

  • Section 3:Determining the Cost of Column and Components

    Step 1. Cost of Column
    Step 2. Cost of the Reboiler
    Step 3. Cost of Condenser
    Step 4. Total Cost of Column

Qualified engineers should consider the following critical steps for distillation column design:

  • Vapor-Liquid Equilibrium
  • Column Operating Objectives
  • Operating Pressure
  • R/Dmin and Nminand Feed stage estimation
  • Diameter and Height of the Column

Vapor-Liquid Equilibrium:

The starting point upon of all column design is on the basis of accurately determining the relative volatility of the key components to be separated. Using a mass and energy balance simulation program, the user must set up the basis of the simulation by selecting an appropriate fluid package and the components present in the feed.

Activity coefficients, estimated by the program or provided by the user, are used to relate non-ideal component interactions.

Column operating objectives:

Column operating objectives are defined by a primary product composition and an optimal recovery of the product from the waste, recycle or less important by-product stream. These specifications should be in terms of the heavy key impurity in the top stream and the light key impurity in the bottom stream.

Operating Pressure:

Once the top and bottom stream compositions are specified, the dew point of the top stream and the boiling point of the bottom stream may be determined at various pressures. An operating pressure should be selected that allows acceptable temperature differences between available utilities because the overhead vapor must be condensed and the bottom liquid reboiled.

If possible, atmospheric or pressure operation of the column is preferred to avoid requiring a vacuum system. Another consideration is also to use component heat sensitivity, which may require lower pressure operation to avoid fouling, product discoloration or decomposition. Often, the relative volatility is also improved at lower pressures.

R/Dmin&Nmin and Feed Stage Estimation:

Using the simulation program, shortcut procedures based upon total reflux operation allow the minimum reflux ratio (R/Dmin) and minimum number of ideal separation stages (Nmin) to be determined.

Using an actual reflux ratio of 1.2 times the minimum reflux ratio will allow an optimal number of stages to be estimated as well as an appropriate feed stage.

Diameter & Height of the column:

At this point, the distillation process is well defined, leaving the column diameter and height to be determined.

The column diameter is chosen to provide an acceptable superficial vapor velocity, or “Fs factor”. This is defined as vapor velocity (ft/sec) times square root of vapor density (lb/ft3), and liquid loading defined as volumetric flow rate (gal/min), divided by the cross sectional area of the column (ft2).

The column internals can be chosen as either trays or packing. Trayed columns must avoid flooding, weeping and down comer backup. Packed columns must avoid flooding, minimum surface wetting and mal-distribution. Table 1 provides a range of typical parameter values for various types of trays.

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